DESCRIPTION: (Applicant's Abstract) Despite the importance of adolescent smoking from a public health perspective (including its association with teenage alcohol and drug involvement), and the evidence from adult twin studies for a strong genetic influence on smoking behavior, there has been little research designed specifically to understand the genetic contribution to risks of becoming a regular tobacco user, and of developing nicotine dependence, and to difficulties with quitting tobacco use. The classical twin study, comparing MZ and DZ twins reared together, is a powerful behavioral genetic design for analyzing the joint effects of genes and environment on behaviors such as the onset of regular tobacco use and transitions in the tobacco habit which are subject to rapid developmental and secular changes. Using a prospective cohort sequential design, we will ascertain cohorts of adolescent same sexed male twins, 11, 13, 15 and 17 years of age, including minority pairs, over a 2-year period, and continue to identify new cohorts of 11-year old twins in years 3, 4 and 5. Each twin and one parent or guardian (whenever possible the biological mother) will be sent a questionnaire and by telephone given an interview that includes assessments of patterns of tobacco use and nicotine dependence, alcohol use and dependence, illicit drug use, other axis I disorders, perceived tobacco, alcohol and drug use and attitudes towards use in peers and parents, and about other pertinent risk factors. There will be brief annual follow-up of the twins. Analyses will include data from an ongoing longitudinal study of female adolescent twins (AA09022; Heath, PI) using similar assessments, allowing us to examine the following issues for cigarette use in adolescent boys compared with girls: (i) genetic influences on tobacco use; (ii) possible mediators of genetic influences on use of and the development of dependence on nicotine; i.e., personality, lifetime depression and anxiety disorders, and adolescent conduct disorder; (iii) the relationship between genetic influences on smoking behavior, and genetic influences on alcohol and illicit drug- related problems; (iv) possible environmental mediators (e.g., perceived smoking in peers); and (v) protective factors (e.g., parental monitoring) which may modify a genetic predisposition to tobacco use and to nicotine dependence in adolescents.